1. Field of the Invention
This invention relates to a radiation image read-out method, a radiation image read-out apparatus and a stimulable phosphor sheet, and more particularly to a radiation image read-out method and a radiation image read-out apparatus for reading out a radiation image stored on a stimulable phosphor sheet by the use of a line sensor, and a stimulable phosphor sheet suitable for the method and the apparatus.
2. Description of the Related Art
When certain kinds of phosphor are exposed to a radiation such as X-rays, they store a part of energy of the radiation. Then when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is generally referred to as “a stimulable phosphor”. In this specification, the light emitted from the stimulable phosphor upon stimulation thereof will be referred to as “stimulated emission”. There has been known a radiation image recording and reproducing system in which a stimulating light beam such as a laser beam is projected onto a stimulable phosphor sheet (a sheet provided with a layer of the stimulable phosphor) which has been exposed to a radiation passing through an object such as a human body to have a radiation image of the object stored on the stimulable phosphor sheet, and the stimulated emission emitted from the stimulable phosphor sheet is photoelectrically detected, thereby obtaining an image signal (a radiation image signal). A radiation image of the object is reproduced as a visible image on the basis of the radiation image signal on a recording medium such as a photographic film or a display such as a CRT. See, for instance, Japanese Unexamined Patent Publications Nos. 55(1980)-12429, 56(1981)-11395 and 56(1981)-11397.
The stimulable phosphor sheet generally comprises a protective layer which protects the surface of the stimulable phosphor layer on the side from which the stimulated emission is detected and a support which supports the stimulable phosphor layer from the side opposite to the protective layer, and is flexible. As a system for reading out radiation image information from the stimulable phosphor sheet, there have been known a point scan system in which the stimulating light is focused in a spot on the surface of the stimulable phosphor sheet, the spot is caused to scan the surface of the stimulable phosphor sheet by the use of a scanning optical system comprising, for instance, a laser and a polygonal mirror while moving the stimulable phosphor sheet in a direction intersecting the direction of scan of the stimulating light spot, and stimulated emission emitted from the points of the stimulable phosphor sheet is led to a photomultiplier by a light guide such as of acrylic resin, and a line scan system in which a linear stimulating light beam is projected onto the stimulable phosphor sheet to irradiate a line-like portion extending in one direction while moving the stimulable phosphor sheet in a direction intersecting said one direction and stimulated emission emitted from the line-like portions is imaged on a line sensor having an array of photoelectric convertor elements.
As a means for imaging the stimulated emission in the line scan system, for instance, an imaging lens comprising an array of a plurality of refractive index profile type lenses may be employed. The imaging lens images the line-like portion of the stimulable phosphor sheet as an erected image of natural size, whereby the line sensor detects the amount of stimulated emission emitted from the line-like portion.
In this case, it is necessary to keep the distance between the surface of the stimulable phosphor layer and the imaging lens in a range where a relation necessary for the imaging lens to image the surface of the stimulable phosphor layer on the line sensor (this relation will be referred to as “imaging relation”, hereinbelow) can be held.
That is, when the distance between the surface of the stimulable phosphor layer and the imaging lens changes, the imaging lens can come to be disabled from imaging a particular part of the surface of the stimulable phosphor layer on a predetermined photoelectric convertor element of the line sensor, which can result in reduction of the amount of stimulated emission detected by the predetermined photoelectric convertor element due to poor light collecting efficiency and/or generation of noise in the radiation image information obtained due to stimulated emission impinging upon a wrong photoelectric convertor element.
This problem may be overcome by supporting the supported surface of the stimulable phosphor layer (the surface opposite to the surface of the stimulable phosphor layer on the side from which the stimulated emission is detected (will be referred to as “the detected surface”, hereinbelow)) by a rigid support so that the detected surface of the stimulable phosphor layer conforms to the shape of the surface of the support, whereby the distance between the detected surface of the stimulable phosphor layer and the imaging lens is kept constant.
However, even if the supported surface of the stimulable phosphor layer is supported by a rigid support, the detected surface cannot always conform to the shape of the surface of the support, and at the same time, the detected surface of the stimulable phosphor layer actually has unevenness and/or undulation and accordingly, it is difficult to move the stimulable phosphor sheet keeping constant the distance between the detected surface of the stimulable phosphor layer and the imaging lens.